Some speculate that this myth dates back to the baby-boom generation, who had worse acne than their parents and also more access to chocolate and fried foods. Wherever this idea came from, it's wrong. Pimples form when oil glands under the skin produce too much of a waxy oil called sebum, which the body uses to keep skin lubricated. But when excess sebum and dead skin cells block pores, that area of the skin gets irritated, swollen, and turns red -- the telltale signs of a pimple. It is unknown why sebaceous glands produce excess sebum, but hormones are the prime suspects, which explains why teenagers are affected more than others. Stress and heredity may also be factors, but chocolate bars and onion rings are off the hook.